A single crystal used as a substrate of semiconductor devices is, for example, a silicon-single crystal. It is mainly produced by Czochralski Method (referred to as CZ method for short hereafter). In recent years, semiconductor devices have come to be integrated higher and devices have come to be decreased in size. Along with the tendency, a problem of Grown-in defects introduced during growth of a single crystal has become more important.
Hereafter, Grown-in defects will be explained with reference to FIG. 8.
Generally, in the case of growing a silicon single crystal, when a growth rate of the crystal (a pulling rate of the crystal) is relatively high, there exist Grown-in defects such as FPD (Flow Pattern Defect) and COP (Crystal Originated Particle), which are considered due to voids consisting of agglomerated vacancy-type point defects, at a high density over the entire radial direction of the crystal. The region containing these defects due to voids is referred to as V (Vacancy) region.
Furthermore, when the growth rate of the crystal is lowered, along with lowering of the growth rate, an OSF (Oxidation Induced Stacking Fault) region is generated from the periphery of the crystal in a ring shape. When the growth rate is further lowered, the OSF ring shrinks to the center of the wafer and disappears. When the growth rate is further lowered, there exist defects such as LSEPD (Large Secco Etch Pit Defect) and LFPD (Large Flow Pattern Defect), which are considered due to dislocation loops consisting of agglomerated interstitial silicon atoms, at a low density. The region where these defects exist is referred to as I (Interstitial) region.
In recent years, a region containing no defects like FPD and COP due to voids as well as no defects like LSEPD and LFPD due to interstitial silicon atoms has been found between the V region and the I region and also outside the OSF ring. This region is referred to as N (Neutral) region. In addition, it has been found that when further classifying the N region, there exist Nv region (the region where a lot of vacancies exist) adjacent to the outside of the OSF ring and Ni region (the region where a lot of interstitial silicon atoms exist) adjacent to the I region, and that when performing thermal oxidation treatment, a lot of oxide precipitates are generated in the Nv region and little oxide precipitates are generated in the Ni region.
Furthermore, it has been found that, after performing the thermal oxidation treatment, there exist a region where defects detected by Cu deposition treatment are particularly generated (hereinafter referred to as Cu deposition defect region) in a portion of the Nv region where oxygen precipitation tends to be generated. And it has been found that the Cu deposition defect region cause degradation of electric property like oxide dielectric breakdown voltage characteristics.
It is considered that introduction amount of these Grown-in defects is determined by a parameter of V/G (mm2/° C.·min) which is a ratio of a pulling rate V (mm/min) when a single crystal is grown and a temperature gradient G (° C./mm) of the crystal in the direction of pulling axis from melting point of silicon to 1400° C. at the vicinity of solid-liquid interface (for example, see V. V. Voronkov, Journal of Crystal Growth, 59 (1982), 625–643). Therefore, a single crystal is grown with controlling V/G to be constant at a determined value, thus a single crystal including desired defect region or desired defect-free region can be produced.
For example, in Japanese Patent Laid-open (Kokai) No. H11–147786, it is disclosed that, when a silicon single crystal is grown, the single crystal is pulled with controlling V/G in a determined range (for example, 0.112–0.142 mm2/° C.·min) at the center of the crystal, thus a silicon single crystal wafer with no defects due to voids and no defects due to dislocation loops can be obtained. Furthermore, in recent years, the demand for a defect-free crystal of N region without Cu deposition defect region has become higher. And it has been demanded a production of a single crystal in which the single crystal is pulled with controlling V/G to be a determined defect-free region with high precision.
Generally, the temperature gradient G of a crystal in the direction of a pulling axis is considered to be determined only by HZ (hot zone: a furnace structure) of an apparatus for pulling a single crystal in which the single crystal is grown. It is known that the temperature gradient G of the crystal becomes lower as the single crystal grows, thus G becomes lower at the end of growing the straight body of the crystal than at the start of growing the straight body of the crystal. However, because it is extremely difficult to change HZ while pulling a single crystal, the temperature gradient G of the crystal isn't controlled. As described above, when a single crystal is grown with controlling V/G to be almost constant at a desired value, a pulling rate V is gradually lowered according to fluctuation (decline) of the temperature gradient G of the crystal as the single crystal grows.
However, even if a single crystal is grown with controlling V/G at a determined value by gradually lowering a pulling rate V as described above, when the defect region enlarging in the radial direction of the actually obtained single crystal is examined entirely in a direction of the crystal growth axis, distribution of the defect region-enlarging in the radial direction is different in former half of the straight body of the single crystal (the vicinity of the shoulder portion) and in latter half of the straight body of the single crystal (the vicinity of the tail portion). Thus there are some cases that a crystal quality (a defect region) determined at the start of growing the straight body of the single crystal can't be maintained entirely in a direction of growth axis.
For example, in order to produce a single crystal of which a whole plane in a radial direction is N region, a pulling rate V is determined at the start of growing the straight body of the single crystal so that V/G is a determined value, and the single crystal is grown with controlling V/G to be constant at a determined value by gradually lowering the pulling rate V according to the fluctuation of the temperature gradient G of the crystal during the growth of the single crystal. In this case, a whole plane in a radial direction of the former half of the straight body of the single crystal is N region. However, OSF region or V region is observed, or I region is observed in some portions of a plane in a radial direction of the latter half of the straight body of the single crystal. Thus, there are some cases that the whole plane in a radial direction of the crystal is not N region.
Especially, in the case of producing a single crystal with a large diameter like 200 mm or more so that the whole plane in a radial direction is N region, or in the case of producing a single crystal with controlling V/G with high precision in a narrower region like Nv region or Ni region in N region without Cu deposition defect region as shown in FIG. 8, it is considered extremely difficult to produce with stability a single crystal with a desired quality entirely in a direction of the crystal growth axis, which is one of the causes that lead to lower yield.